Visible Pulses Research Articles

Femtosecond Dynamics of Spin-Polarized Electrons in Topological Insulators

A fast control of spins is a major quest in spintronic systems. Ultrashort light pulses have been utilized to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers. Recently, three-dimensional topological insulators have received attention due to the existence, within the insulating band gap of bulk states, of spin-polarized surface states that are protected from backscattering by time-reversal symmetry. We studied sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of Bi2Te3, employing circular-polarized light in time- and angle-resolved photoemission spectroscopy. Noncollinear optical parametric amplification and several nonlinear optical processes result in tunable, ultrashort visible pump pulses with a duration of 30 fs and 1.8 eV energy and ultraviolet probe pulses with about 60 fs duraction and about 6 eV energy. The stable optical setup and the high repetition rate of an Yb-laser source gives a high signal-to-noise ratio in our photoemission process. The 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight energy analyzer, provides the opportunity to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism allows access to the spin state of the photoemitted electrons. A signature of femtosecond unpolarized bulk bands dynamics appears in the presence of spin-polarized electrons of the surface states. This helps distinguish the bulk and surface contributions in the spin-electronic current.

Investigation for Ignition of ADN-based Ionic Liquid with Visible Pulse Laser

Investigation for Ignition of ADN-based Ionic Liquid with Visible Pulse Laser

High-resolution broadband sum frequency generation vibrational spectroscopy using intrapulse interference.

We theoretically propose a new approach to obtain high-spectral-resolution sum frequency generation (SFG) vibrational spectra using intrapulse interference. By introducing a π-step phase modulation to the broadband 800 nm pulse, the broadband 800 nm laser pulse splits into two distinguishable pulses in the time domain with a fixed time delay. The resolution of the intrapulse interference SFG can be better than 1 cm-1 and is limited only by the spectral resolution of the spectrometer. This approach can accurately retrieve the amplitude and the relative phase of vibrational peaks. Additionally, the sensitivity of SFG is enhanced by adopting femtosecond IR and femtosecond visible pulses.

HV discharge acceleration by sequences of UV laser filaments with visible and near-infrared pulses

We investigate the triggering and guiding of DC high-voltage discharges over a distance of 37 cm by filaments produced by ultraviolet (266 nm) laser pulses of 200 ps duration. The latter reduce the breakdown electric field by half and allow up to 80% discharge probability in an electric field of 920 kV m–1. This high efficiency is not further increased by adding nanosecond pulses in the Joule range at 532 and at 1064 nm. However, the latter statistically increases the guiding length, thereby accelerating the discharge by a factor of 2. This effect is due both to photodetachment and to the heating of the plasma channel, that increases the efficiency of avalanche ionization and reduces electron attachment and recombination.

Carrier frequency tuning of few-cycle light pulses by a broadband attenuating mirror.

We demonstrate the performance of a novel multilayer dielectric reflective thin-film attenuator capable of reshaping the super-octave spectrum of near-single-cycle visible laser pulses without deteriorating the phase properties of the reflected light. These novel broadband attenuating mirrors reshape in a virtually dispersion-free manner the incident spectrum such that the carrier wavelength of the reflected pulses shifts from ∼700 nm (Eγ=1.77 eV) to ∼540 nm (Eγ=2.25 eV) or beyond while maintaining their initial near-single-cycle pulse duration. This constitutes a viable approach to convert a number of established few-cycle ultrafast laser systems into sources with a selectable excitation wavelength to meet the requirements of single-color/multicolor high temporal resolution spectroscopic experiments.

Streak Camera for Strong-Field Ionization.

Ionization of an atom or molecule by a strong laser field produces suboptical cycle wave packets whose control has given rise to attosecond science. The final states of the wave packets depend on ionization and deflection by the laser field, which are convoluted in conventional experiments. Here, we demonstrate a technique enabling efficient electron deflection, separate from the field driving strong-field ionization. Using a midinfrared deflection field permits one to distinguish electron wave packets generated at different field maxima of an intense few-cycle visible laser pulse. We utilize this capability to trace the scattering of low-energy electrons driven by the midinfrared field. Our approach represents a general technique for studying and controlling strong-field ionization dynamics on the attosecond time scale.

Visible continuum pulses based on enhanced dispersive wave generation for endogenous fluorescence imaging.

In this study, we demonstrate endogenous fluorescence imaging using visible continuum pulses based on 100-fs Ti:sapphire oscillator and a nonlinear photonic crystal fiber. Broadband (500-700 nm) and high-power (150 mW) continuum pulses are generated through enhanced dispersive wave generation by pumping femtosecond pulses at the anomalous dispersion region near zero-dispersion wavelength of high-nonlinear photonic crystal fibers. We also minimize the continuum pulse width by determining the proper fiber length. The visible-wavelength two-photon microscopy produces NADH and tryptophan images of mice tissues simultaneously. Our 500-700 nm continuum pulses support extending nonlinear microscopy to visible wavelength range that is inaccessible to 100-fs Ti:sapphire oscillators and other applications requiring visible laser pulses.

Room‐Temperature Processing of Printed Oxide FETs Using Ultraviolet Photonic Curing

Oxide semiconductors are highly suitable materials for solution‐processed/printed electronics (PE); the growing interest in them can be related to their excellent intrinsic properties, such as high mobility, optical transparency, thermal and environmental stability, and so on. However, high process temperatures remain as the foremost challenge that may limit the compatibility of printed oxide electronics to inexpensive flexible substrates. Here, the possibility of using photonic curing methods to lower the process temperature is investigated, even down to room temperature (RT). Two distinct ultraviolet (UV) curing techniques, involving UV–visible light pulses and continuous‐wave UV lasers are exercised and compared. Combining UV curing with oxide nanoparticulate channel layer and electrolytic gate insulators, it is demonstrated that high performance field‐effect transistors (FETs) with device mobility as high as 12 cm2 V−1 s−1 can be processed entirely at room temperature and realized on plastic substrates. The fabrication steps include printing of a heavily stabilized semiconducting nanoparticulate channel layer, followed by decomposition and removal of the semi‐insulating polymer ligands using UV‐photon energies. The curing process is found to be fast and high‐throughput manufacturing technique compatible. At the same time, the energy requirement to remove the polymer stabilizers is insignificant, thereby ensuring no temperature rise of the parent substrates.

Visible ns‐pulse laser oscillation in Pr‐doped double‐clad structured waterproof fluoride glass fibre with SESAM

A visible Q-switched pulse train at 639 nm was successfully generated in a Pr-doped double-clad structured waterproof fluoride glass fibre with a semiconductor saturable absorber mirror. The slope efficiency was calculated as 36.3%. The pulse duration and the radio frequency were measured as 270 ns and 107 kHz at 596 mW of absorbed power, respectively. The pulse energy and the pulse peak power were calculated as 1.32 μJ and 4.87 W, respectively.

Real-time observation of interfragment vibration and charge transfer within the TCNQF4 dimer

The organic electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), are widely used to synthesize multivalent charge-transfer complexes. Their corresponding radical anion complexes have specific electronic, optical, and magnetic characters that have attracted attention in the field of materials development. In the present work, we have performed time-resolved absorption spectroscopy of the (Na+)2(TCNQF4−)2 radical anion complex to elucidate the reaction dynamics ([TCNQF4−]2→TCNQF4+TCNQF42−). A visible sub-10fs laser pulse was used to observe the ultrafast dynamics of the charge transfer and real-time change of the molecular structure during the reaction. Unlike our previous work on the (Na+)2(TCNQ−)2 radical anion complex, the interfragment vibrational mode was clearly observed for (Na+)2(TCNQF4−)2. The difference in the intermolecular interaction is considered to reflect the difference in the π-π interaction and/or the difference in the flexibility of the molecular plane predicted by theoretical calculations.

Mid-Infrared photorefractivity with non-degenerate two-photon absorption

Photorefractive grating recording with mid-infrared sub-picosecond light pulses (1.6 μm and longer wavelengths) might be possible in LiNbO3 crystals if the interaction area is illuminated additionally with pulses of visible light. Two processes can enable the experimentally observed MIR recording: One is related to the direct photoexcitation of a free electron via nonlinear absorption of two photons with different frequencies, one in the MIR and one in the visible range. The alternative possibility consists in an enhancement, caused by the diffraction of a visible pulse, of a very weak space-charge grating seeded solely by MIR radiation.

Compact, high-repetition-rate source for broadband sum-frequency generation spectroscopy

We present a high-efficiency optical parametric source for broadband vibrational sum-frequency generation (BB-VSFG) for the chemically important mid-infrared spectral range at 2800–3600 cm−1 to study hydrogen bonding interactions affecting the structural organization of biomolecules at water interfaces. The source consists of a supercontinuum-seeded, dual-beam optical parametric amplifier with two broadband infrared output beams and a chirped sum-frequency mixing stage providing narrowband visible pulses with adjustable bandwidth. Utilizing a pulse energy of only 60 μJ from a turn-key, 1.03-μm pump laser operating at a repetition rate of 100 kHz, the source delivers 6-cycle infrared pulses at 1.5 and 3.2 μm with pulse energies of 4.6 and 1.8 μJ, respectively, and narrowband pulses at 0.515 μm with a pulse energy of 5.0 μJ. The 3.2-μm pulses are passively carrier envelope phase stabilized with fluctuations at the 180-mrad level over a 10-s time period. The 1.5-μm beamline can be exploited to deliver pump pulses for time-resolved studies after suitable frequency up-conversion. The high efficiency, stability, and two orders of magnitude higher repetition rate of the source compared to typically employed systems offer great potential for providing a boost in sensitivity in BB-VSFG experiments at a reduced cost.

Characterization of induced nanoplasmonic fields in time-resolved photoemission: A classical trajectory approach applied to gold nanospheres

Attosecond time-resolved spectroscopy has been shown to be a powerful method for examining the electronic dynamics in atoms, and this technique is now being transferred to the investigation of elastic and inelastic scattering during electron transport and collective electronic (plasmonic) effects in solids. By sampling over classical photoelectron trajectories, we simulated streaked photoelectron energy spectra as a function of the time delay between ionizing isolated attosecond extreme ultraviolet (XUV) pulses and assisting infrared or visible streaking laser pulses. Our calculations comprise a sequence of four steps: XUV excitation, electron transport in matter, escape from the surface, and propagation to the photoelectron detector. Based on numerical applications to gold nanospheres of 5- and 50-nm radius, we investigate streaked photoemission spectra with regard to (i) the nanoparticle's dielectric response to the electric field of the streaking laser pulse, (ii) relative contributions to photoelectron release from different locations on the surface and inside the nanoparticle, (iii) contributions of photoemission from the Fermi level only versus emission from the entire occupied conduction band, and (iv) their fidelity in imaging the spatiotemporal distribution of the induced plasmonic field near the particle's surface.

Enhancing nonlinear energy deposition into transparent solids with an elliptically polarized and mid-IR heating laser pulse under two-color femtosecond impact

We report on an enhancement of deposited energy density of up to 10 kJ cm−3 inside transparent solids (fused silica and quartz) from using two-color µJ energy level tightly focused (NA = 0.5) co-propagating linearly polarized seeding (visible, 0.62 µm) and elliptically polarized heating (near-IR, 1.24 µm) femtosecond laser pulses. The rise in temperature under constant volume causes pressure of up to 12 GPa. It has been shown experimentally and theoretically that the production of seeding electrons through multiphoton ionization by visible laser pulse paves the way for controllability of the energy deposition and laser-induced micromodification via carrier heating by delayed infrared laser pulses inside the material. The developed theoretical approach predicts that the deposited energy density will be enhanced by up to 14 kJ cm−3 when using longer (up to 5 µm) wavelengths for heating laser pulses inside transparent solids.

Vibrational Two-Photon Emission from Coherently Excited Solid Parahydrogen.

We report the observation of two-photon emission from a coherently excited vibrational state of solid parahydrogen, which is also a known quantum solid. Coherence between the ground and the excited states is prepared by stimulated Raman scattering using two visible laser pulses. The two-photon emission is triggered by another mid-infrared laser pulse. It was observed that the two-photon emission persists even when the trigger pulse is injected long after the excitation. This is due to the long decoherence time of the vibrational states of solid parahydrogen. It is found that the emission intensity increases even after the excitation pulses pass through the target completely. This coherence development is highly suppressed at high target temperatures and high residual orthohydrogen concentrations. Effects of target annealing and laser-induced damage on the target are also observed.

Transient absorption imaging of hemes with 2-color, independently tunable visible-wavelength ultrafast source.

Pump probe microscopy is a time-resolved multiphoton imaging technique capable of generating contrast between non-fluorescent pigments based on differences in excited-state lifetimes. Here we describe a fiber-based ultrafast system designed for imaging heme proteins with an independently-tunable pulse pair in the visible-wavelength regime. Starting with a 1060 nm fiber amplifier (1.3 W at 63 MHz, 140 fs pulses), visible pulses were produced in the vicinity of 488 nm and 532 nm by doubling the output of a short photonic crystal fiber with a pair of periodically-poled lithium niobate crystals, providing 5-20 mW power in each beam. This was sufficient for acquiring transient absorption images from unstained cryosectioned tissue.

Controlled energy deposition and void-like modification inside transparent solids by two-color tightly focused femtosecond laser pulses

We report a bulk void-like micromodification of fused silica using two-color μJ-energy level tightly focused (NA = 0.5) co-propagating seeding (visible, 0.62 μm) and heating (near-IR, 1.24 μm) femtosecond laser pulses with online third harmonic diagnostics of created microplasmas as well as subsequent laser-induced void-like defects. It has been shown experimentally and theoretically that production of seeding electrons through multiphoton ionization by visible laser pulses paves the way for controllability of the energy deposition and laser-induced micromodification via carrier heating by delayed infrared laser pulses inside the material. Experimental results demonstrate wide possibilities to increase the density of energy deposited up to 6 kJ cm−3 inside the dielectric by tight focusing of two color fs-laser pulses and elliptical polarization for infrared heating fs-laser pulses. The developed theoretical approach predicts the enhancement of deposited energy density up to 9 kJ cm−3 using longer (mid-IR) wavelengths for heating laser pulses.

Intensity dependences of the nonlinear optical excitation of plasmons in graphene.

Recently, we demonstrated an all-optical coupling scheme for plasmons, which takes advantage of the intrinsic nonlinear optical response of graphene. Frequency mixing using free-space, visible light pulses generates surface plasmons in a planar graphene sample, where the phase matching condition can define both the wavevector and energy of surface waves and intraband transitions. Here, we also show that the plasmon generation process is strongly intensity-dependent, with resonance features washed out for absorbed pulse fluences greater than 0.1 J m-2 This implies a subtle interplay between the nonlinear generation process and sample heating. We discuss these effects in terms of a non-equilibrium charge distribution using a two-temperature model.This article is part of the themed issue 'New horizons for nanophotonics'.

Broadband atomic-layer MoS2 optical modulators for ultrafast pulse generations in the visible range.

Visible lasers are a fascinating regime, and their significance is illustrated by the 2014 Noble prizes in physics and chemistry. With the development of blue laser diodes (LDs), the LD-pumped solid-state visible lasers become a burgeoning direction today. Constrained by the scarce visible optical modulators, the solid-state ultrafast visible lasers are rarely realized. Based on the bandgap structure and optoelectronic properties of atomic-layer MoS2, it can be proposed that MoS2 has the potential as a visible optical modulator. Here, by originally revealing layer-dependent nonlinear absorption of the atomic-layer MoS2 in the visible range, broadband atomic-layer MoS2 optical modulators for the visible ultrafast pulse generation are developed and selected based on the proposed design criteria for novel two-dimensional (2D) optical modulators. By applying the selected MoS2 optical modulators in the solid-state praseodymium lasers, broadband mode-locked ultrafast lasers from 522 to 639 nm are originally realized. We believe that this Letter should promote the development of visible ultrafast photonics and further applications of 2D optoelectronic materials.

Studies of electrochemical interfaces by broadband sum frequency generation

We present a perspective on the use of potential-dependent broadband multiplex vibrational sum-frequency generation spectroscopy (hereafter SFG) to study electrochemical systems. In SFG, a broadband mid-infrared (IR) pulse is combined with a narrowband visible pulse, generating a pulse at the sum frequency, which contains a spectrum that, due to the principles of nonlinear optics, originates solely from the electrified interface. Our SFG spectrometer can obtain one hundred or more spectra during a routine cyclic voltammetry (CV) measurement. We used SFG to study a model for a Li-ion battery anode and a low-overpotential CO2 reduction reactor based on a room-temperature ionic liquid (RTIL). SFG spectra from these complex systems were difficult to interpret, in part due to the organic electrolytes that have a forest of infrared vibrational transitions. Here we discuss a rubric for experimental measurement and interpretation with such systems. We describe the electrified interface, or double layer, as consisting of the electrode surface, a region of adsorbed molecules and an outer diffuse electric double layer. We combined resonant SFG, where the IR pulses were tuned to vibrational transitions of adsorbates, with nonresonant SFG where the IR pulses were tuned away from all vibrational transitions. The former provides information about chemistry on the electrode surface and the latter about the potential-dependent response of the double layer. We show how this rubric can be used to understand solid-electrolyte interphase formation on a model for the Li-ion electrode, and how low-overpotential CO2 reduction on Ag is controlled by potential-driven structural transitions of the RTIL electrolyte.